Advisor-Built Solo 401k: Open a Solo 401(k) in 2026 and Still Fund a 2025 Mega Backdoor Roth

Many self-employed professionals and advisors assume that once the calendar year ends, opportunities to maximize retirement contributions are gone. Fortunately, that is not always true.

Thanks to changes under SECURE Act 2.0, it is still possible to open a Solo 401(k) in 2026 and make voluntary after-tax contributions for the 2025 tax year, which can then be converted using the Mega Backdoor Roth strategy—if done correctly.

This article explains who qualifies, why it works, and how My Solo401k Financial supports the process, including custody at Fidelity, Schwab, Altruist, or an advisor’s preferred broker-dealer.

What Is the Mega Backdoor Roth Solo 401(k)?

The Mega Backdoor Roth is a strategy that allows self-employed individuals to move significantly more money into Roth accounts than standard Roth IRA limits allow.

It works by:

Making voluntary after-tax Solo 401(k) contributions
Converting those contributions to a Roth Solo 401(k) or Roth IRA
Allowing future earnings to grow tax-free

Because after-tax contributions are not deductible, the principal converts tax-free, and only earnings (if any) are taxable at conversion—making timing critical.

The Key Insight: You Can Open a Solo 401(k) in 2026 for 2025

Under updated IRS rules, voluntary after-tax Solo 401(k) contributions may be made by the business tax return due date, including extensions, even if the plan itself is adopted in the following year.

That means:

A Solo 401(k) can be opened in 2026
Contributions can still be designated for tax year 2025
The Mega Backdoor Roth strategy remains available—if deadlines are met

This is especially powerful for advisors working with high-income self-employed clients who missed the traditional December 31 setup deadline.

Contribution Deadlines by Entity Type

Deadlines vary based on how the business is taxed:

Sole Proprietors & Single-Member LLCs

Plan setup deadline: April 15, 2026

Eligible for:
- Employee deferrals
- Employer contributions
- Voluntary after-tax contributions
With extension: October 15, 2026
Eligible for:
- Employer contributions
- Voluntary after-tax contributions

S-Corporations

Setup deadline: March 15, 2026
With extension: September 15, 2026
Eligible for:
- Employer profit sharing
- Voluntary after-tax contributions
The plan had to be adopted by 12/31/2025 for Employee deferrals

Partnerships

Same deadlines as S-corps
Voluntary after-tax contributions still allowed
Employee deferrals not available if plan is first adopted in 2026

Why Fidelity & Schwab Alone Is Not Enough

While Fidelity and Schwab offers Solo 401(k) accounts, their prototype Solo 401(k)s do not support Mega Backdoor Roth features such as:

Voluntary after-tax contributions
In-service Roth conversions

This is where My Solo401k Financial comes in.

My Solo401k provides custom Solo 401(k) plan documents that:

Explicitly allow voluntary after-tax contributions
Permit Roth conversions
Still allow assets to be custodied at Fidelity, Schwab or another broker-dealer of choice

How the Mega Backdoor Roth Conversion Works

Once the after-tax contribution is made:

Funds are moved from the after-tax Solo 401(k) account
Converted immediately to:
- A Roth Solo 401(k) (in-plan conversion), or
- A Roth IRA (rollover conversion)
Conversion is reported on Form 1099-R, which MySolo401k Financial prepares as part of its compliance services

Immediate conversion is recommended to minimize taxable earnings.

Spouses Can Double the Strategy

When both spouses work in the same self-employed business:

One Solo 401(k) plan is sponsored by the business
Each spouse has separate participant accounts
Each spouse can execute their own Mega Backdoor Roth conversion

This effectively doubles the amount that can be moved into Roth accounts for the same tax year

Why Advisors Use MySolo401k Financial

Advisors value MySolo401k Financial because:

The advisor retains asset custody and investment oversight
The client in conjunction with her advisor chooses Fidelity, Schwab, Altruist, or another custodian

My Solo 401k Financial handles:
- Plan documents
- Contribution calculations
- Form 1099-R
- Form 5500-EZ when required
No conflict with broker-dealer relationships